There are also public domain versions of
OS/VS1
, VM/370
and DOS/VS
. I believe source code is available for many if not all of the public domain
operating systems, which represents a tremendous educational opportunity.

For a real bargain, you can order the public domain operating systems from
the CBT tape
site. CBT also hosts some older versions of the CBT tape from the
era when these operating systems were current.

Finally, there are licensed operating systems such as OS/390 and z/OS. While
they can run under Hercules, if you are interested in running them I recommend
you contact IBM
for their licensing terms before doing so.